Langmuir 1993,9, 1397-1403
1397
Application of Surface-Enhanced Raman Spectroscopy to _ _ Organic Electrocatalytic Systems: Decomposition and ElectPooxidation of Methanol and Formic Acid on Gold and Platinum-Film Electrodes Yun Zhang and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Received October 26,1992. In Final Form: February 15,1993
The nature of adsorbed speciesformed by decompmition of methanol and formic acid under voltammetric conditions on gold and platinum-coatedgold electrodes in acidic and alkaline media has been explored by means of surface-enhancedRaman spectroscopy (SERS). Sequences of SER spectra were recorded during potential-sweep cycles to correlate the potential-dependent surface speciation with reactant electrooxidation and other voltammetricfeatures. Despite the observed absence of methanol electrooxidation on gold in perchloric acid, decomposition to yield adsorbed -CHO,-COH, and 72 (0,O) formate, as well as CO, was detected from the SER spectra. In contrast to the apparent inability of these species to undergo electrooxidation under such conditions, adsorbed CO formed from solution CO is seen to undergo remarkably facile electrooxidation on gold. The latter observation is apparently connected with the formation of CO bound to partially oxidized gold sites even at low potentials, as observed by SERS. Such reactive speciesare not formed by methanol decomposition. Formic acid,however,readily undergoes electrooxidation on gold in perchloric acid; adsorbed formate rather than CO is identified by SERS as the reaction intermediate. Methanol, but not formate,undergoes electrooxidation in 0.1 M KOH, the former yields adsorbed CO extensively at higher concentrations. Despite facile electrooxidation of methanol on the platinum films in acid, little adsorbed CO is discerned to be present from both SERS and surface infrared spectroscopy. The former techniquesuggestsinstead the predominant formationof an acetylenic species. Some virtues and limitations of SERS for deducing the nature of adsorbed species in organic electrocatalyticprocesses are noted in the light of these findings. offering reasonablesensitivity,these measurements usually yield differential (bipolar) bands rather than the desired The electrocatalytic oxidations of small organic moleabsolute spectra and are obtained under potentiodynamic cules, especially methanol and other primary alcohols, at conditions that are not necessarily relevant to electrocatransition-metal and related surfaces have long been a talysis. Severalalternative infrared measurement schemes subject of widespread practical as well as fundamental have been pursued with the aim of circumventing these interest given their importance in fuel-cell technology.' difficulties,includingthe use of a flow cell? p/s polarization The central issue for such systems is the nature of the modulation,7 and potential-difference methods utilizing adsorbed species present under reaction conditions and, only a single potential step or sweep during the spectral particularly, their role as reaction intermediates or poisons. acquisition.8 These tactics can enable irreversible as well Over the last ten years or so, significant insight has been as reversible potential-induced changes in the surface obtained on this question, primarily from in situ infrared composition to be followed during the evolution of the spectroscopy but also from on-line mass spectrometry.lP2 electrocatalytic process, thereby yielding information on The former technique has clearly identified the common the nature of adsorbates present under reactive electropresence of adsorbed carbon monoxide on platinum-group chemical conditions. Nevertheless, the restricted fremetals formed by dissociative chemisorption of not only quency range amenable to surface infrared spectroscopy methanol3 but also a range of other oxygen-containing combined with the inevitable limitations arising from organic molecules in aqueous solution.4 Some evidence sensitivity, surface selection rules, and so on, make it very for the presence of other adsorbed fragments, such as COH, desirable to also obtain adsorbate vibrational information has also been obtained from infrared s p e c t r o ~ c o p y . ~ ~by ~ ~other ~ ~ methods. Despite these advances, a number of unsettled (and Besidesinfrared spectroscopy,surface-enhanced Raman unsettling) questions remain. A difficulty with many such scattering (SERS)provides a viable in situ means of in situ infrared measurements is that they commonly utilize obtainingvibrational spectra at metal-solution interfaces? repeated potential modulation in order to minimize bulk Most importantly, the very large surface enhancements solution interferences and optimize signal-to-noise, recharacteristic of the latter technique enable vibrational quired for detection of weakly absorbing adsorbates? While information to be obtained for a variety of adsorbates over wide frequency ranges. The key advantages of SERS as compared with surface-infrared spectroscopyare the usual (1) For overviews, see: (a) VanderNoot, T.; Parsons, R. J. Electroanal. Chem. 1988,257,9. (b) Leger, J.-M., Lamy, C. Ber. Bunsen-Ges. Phys. lack of solvent and other bulk solution interferences and C k m . 1990,94,1021. Iwosita,T. In Aduances in Electrochemical Science the ability to obtain sensitive spectral information even and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH Publishers:
Introduction
Weinheim and New York, 1990; Vol. 1, p 127. (2) Iwosita, T.; Nart, F. C.; Lopez, B.; Vielstich, W. Electrochim. Acta 1992,37, 2361. (3) Kunimatau, K. Ber. Bunsen-Ges. Phys. Chem. 1990,94,1025. (4) Leung, L.-W. H.; Weaver, M. J. Langmuir 1990,6, 323. (5) Nichols, R. J.; Bewick, A. Electrochim. Acta 1988,33, 1691. (6) For example, see Beden, B.; Juanto, S.; Leger, J. M.; Lamy, C. J. Electroanal. C&m. 1987, 238, 323.
~~~
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(7) Kunimatsu, K.; Kita, H. J . Electroanal. Chem. 1987,218, 155. (8) Corrigan, D. S.; Weaver, M. J. J . Electroanal, Chem. 1988, 241, 143. (9) For recent overviews, see (a) Heater, R. E. In Comprehensiue Chemical Kinetics; Compton, R. G., Hamnett, A., Eds.; Elsevier: Amsterdam, 1989; Vol. 29, Chapter 2. (b) Otto, A. J. Raman Spectrosc. 1991, 22, 743.
0743-7463/93/2409-1397$04.00/00 1993 American Chemical Society
1398 Langmuir, Vol. 9, No. 5, 1993
down to low frequencies, 100-700 cm-l, where surfaceadsorbate vibrations are commonly located. In addition, the recent advent of charge-coupled device (CCD) detectors enables rapid real-time sequences of SER spectra to be acquired during the voltammetric or other electrochemical perturbations commonly utilized for the elucidation of electrode processes. Unfortunately, two well-documented limitations of SERS have largely discouraged bona fide applications of the technique to electrocatalytic systems. The fiist is the need to roughen the metal surface in order to generate the SERS effect. While the degree of roughness required is relatively mild (in a macroscopic sense), the increased attention being paid currently to adsorption on ordered monocrystalline electrodeshas yielded a greater emphasis on techniques such as infrared spectroscopy that are also applicable to such systems.1° Nevertheless, the practical electrocatalytic importance of polycrystalline surfaces provides at least one persuasive reason to continue to explore vigorously their properties. The second and perhaps more critical restriction is the limitation of the SERS effect (for most practical purposes) to the “coinage metals”,copper, silver, and gold. While the large majority of SERS studies have involved silver, we have preferred to utilize gold surfaces. There are several reasons fqr the latter choice, including the inertness and wide polarizable potential ranges achievable on gold even in aqueous media. In addition,we demonstratedsome time agothat the SERS effect can be extended straightforwardly to a variety of other electrode materials, including transition metals and oxides, by deposition as ultrathin films on a SERS-active gold substrate.12J3 One consequenceof this finding is that in suitable cases SERScan now be utilized in a more broadbased fashion to explore the role of adsorption in catalytic systems of true practical importance. We have exploited this approach to examine heterogeneouscatalyticreactions in high-pressure gas phase14 as well as electrochemical environments.15J6 We have recently undertaken a systematic examination of the voltammetric electrooxidation of small organic molecules on gold and transition-metal-coated electrodes using such real-time SERS tactics. Described herein are some pertinent results along these lines for the adsorption and electrooxidation of methanol and formic acid in aqueous media on gold and platinum-coated gold electrodes. Gold is commonly considered to be a poor electrocatalyst in comparison with platinum-group metals. Nonetheless,gold is capable of electrooxidizingformic acid and especially carbon monoxide at low overpotentials in acidic perchlorate media” and is known to be a relatively ubiquitous catalyst for organic electrooxidationsin alkaline (10) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5391. (11) (a) Gao, P.; Patterson, M. L.; Tadayyoni, M. A.; Weaver, M. J. Langmuir 1985,I , 173. (b)Gao, P.; Gosztola,D.; Leung, L.-W. H.; Weaver, M. J. J. Electroanal. Chem. 1987, 233, 211. (12) (a) Leung, L.-W. H.; Weaver, M. J. J. Am. Chem. SOC.1987,109, 5113. (b)Leung, L-W. H.; Weaver, M. J. Longmuir 1988,4, 1076. (13) (a) Desilvestro, J.; Corrigan, D. A.; Weaver, M. J. J. Phys. Chem. 1986,90,6408. (b) Gosztola, D.; Weaver, M. J. Langmuir 1989,5776. ( c ) Gosztola, D.; Weaver, M. J. J. Electroanal. Chem. 1989,271, 141. (14) (a) Wilke, T.; Gao, X.; Takoudis, C. G.; Weaver, M. J. Langmuir 1991, 7, 714. (b) Wilke, T.; Gao, X.; Takoudis, C. G.; Weaver, M.J. J. Catal. 1991, 130, 62. (c) Tolia, A.; Wilke, T.; Weaver, M. J.; Takoudis, C. G. Chem. Eng. Sci. 1992,47, 2781. (15) (a) Gao, P.; Gosztola, D.; Weaver, M. J. J. Phys. Chem. 1988,92, 7122. (b) Gao, P.; Gosztola, D.; Weaver, M. J. J. Phys. Chem. 1989,93, 3753. (16) Gao, X.; Zhang, Y.; Weaver, M. J. Langmuir 1992,8, 668. (17) (a) Hamelin, A.; Ho, Y.;Chang, S A . ; Gao, X.; Weaver, M. J. Langmuir 1992,8,975. (b) Chang, S.-C.; Hamelin, A.; Weaver, M. J. J. Phys. Chem. 1991,95,5560. (c) Chang, S.-C.; Hamelin, A.; Weaver, M. J. Surf. Sci. 1990, 239, L543.
Zhang and Weaver
solution.18 While motivated originallyby the convenience of performing SERS on unmodified gold and the need to compare the data with that obtained for platinum-coated surfaces, the present resulta uncover an unexpected ability on gold to engender dissociative chemisorption of methanol. The findings demonstrate the type of additional insight into surfacespeciation in electrocatalytic chemistry that can be obtained by SERS, along with some limitations of the approach for this purpose. Experimental Section The 0.1 M HC104 supporting electrolyte was prepared from 70% HCIOl (GFS Chemicals, double distilled from Vycor) and 0.1 M KOH from ultrapure KOH pellets (Johnson Matthey). Methanol was purchased from Fisher Scientificand distilled prior to use. Formic acid (Mallinckrodt) was usedas received. Carbon monoxide (99.8%) was procured from Matheson. All solutions were prepared by using water purified by a Milli-Q plus system (Millipore). The working electrode was a 4mm diameter gold disk sheathed in Teflon,which was mechanicallypolished with0.3-pm aluminum on a microcloth (Buehler) and rinsed thoroughly with water. The gold surface was then electrochemicallyroughened in 0.1 M KCl to yield stable SERS activity as described previously.llb The platinum-coatedsurfaceswere prepared by electrodepositiononto a freshly roughened gold electrode essentially as described in ref 12a. It was found that the deposition of 2-3 equivalent monolayed of platinum was necessary to suppress spectral features characteristic of substrateadsorbate bonding, using CO as a probe molecule. Progressiveattenuation of the SERS signals arising from adsorbate bound to the overlayermaterial was found for Pt layers thicker than about 3 equivalent monolayers (cf. ref 12a). A PAR Model 173/179potentiatat with a Model 175waveform generator was used for potential control. The glass electrochemical cell used for combined electrochemical/SERS measurements was of conventional design. The central cylindrical working compartment containing the gold electrode and a platinum wire counter electrode was separated from the reference compartment containing a saturated calomel electrode (SCE) by a fine glass frit. The Raman excitation source was a Spectra-Physics Model 165 Kr+ laser operated at 647.1 nm and at a power of -40 mW on the sample. The Raman-scattered light was collected with a 50 mm diameter camera lens (DO Industries Model DO-5095) and focused into a SPEX Model 1877Triplemate spectrometer. As in ref 16, the latter was equipped with a Photometrics PM 512 CCD detector which was cooled to a temperature of 163 K. The detector was operated by a Photometrics CC200 camera controller which was interfaced with a Zenith 386 computer for data acquisition and storage. The spectrometer configuration utilized 600 grooves mm-l ruled gratings in both the filter and spectrograph stage. All spectra were recorded at room temperature. All potentials quoted here are with respect to the SCE.
Results and Discussion Methanol on Gold. The basic experimental tactic utilized here involves obtaining sequences of SER spectra during cyclic voltammetric potential excursions, usually initially in the positive direction at 10 mV s-l. The key virtue of this procedure is that the evolution of the surface vibrational state can be examined simultaneously with the acquisition of voltammetric information. Figures 1 and 2 display selected members of typical potentialdependent SER spectralsequencesobtained in this fashion on gold in aqueous 0.1 M HClOr containing 0,5 and 5 mM methanol, respectively. The electrode potentials noted alongside each spectrum are the values at the start of each 8-13spectral acquisition. Since the voltammetric sweep (18) Beden, B.; Cetin, I.; Kahyaoglu, A.; Takky, D.; Lamy, C. J. Cotal. 1987, 104, 37.
Langmuir, Vol. 9, No.5, 1993 1399
Decomposition of Methanol on Gold
ndw
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Figure 1. Selected members of potential-dependent SER spectral sequence obtained on gold during voltammogram at 10 mV a-1 for 0.5 mM methanol in 0.1 M HClOd from -0.3 to 1.5 V vs SCE and return. Each spectrum shown (in upward-going sequence) was acquired over 8 a, starting at the potentials indicated alongside the sequence in the far left-hand column.
toward higher potentials and during the return portion of the voltammetric sweep. This band can be assigned straightforwardly to the C-O stretch (YCO) of terminally adsorbed carbonmonoxide from the near-identicalfeature obtained upon adsorption of solution CO (vide infra).IQ The weaker feature seen at 465-470 cm-’ (Figures 1and 2) is also consistent with the presence of adsorbed CO, assigned specificallyto the surface-carbon (UM-CO) stretch. Several other bands are also apparent in these spectra that signal the presence of other chemisorbed residues. At low methanol concentrations (51mM) a band at 1640 cm-l and a weaker feature at about 1270cm-l are commonly present, again being evident increasingly toward more positive potentials. Weak bands between 1600 and 1700 cm-I have apparently been observed on several occasions from low methanol concentrations on platinum electrodes by means of potential-modulation infrared spectroscopy and attributed to the carbonyl stretch for adsorbedformyl (-CH0).6120 The 1270-cm-l band observed in Figure 1can be assigned to a C-O stretch for adsorbed -C-OH; similar features have also been reported for the methanol/ platinum system from in situ infrared spectro~copy.5>~~ One can envisage dissociation of methanol on gold as proceedingvia initial C-H bond cleavageto yield adsorbed CH20H followed by stepwise dissociation CH@H
-
-CHm-
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/ -CHo \\ \\ 4 - 0 H / co
(”
Evidence that the dissociative chemisorption of alcohols to yield CO at metal-solution interfaces proceeds via ratedeterminingC-H bond cleavage has been obtained directly for Rh(100) from the observation of a deuterium isotope effect on the formation kinetics by means of infrared spectroscopy.22 Isotopic evidence for initial scission of a C-H bond has also been found for methanol decomposition on platinum electrode^.^^ This situation is quite different from methanol decomposition on transition metals in vacuum, where a methoxy intermediate is commonly formed involvingrate-determining0-H bond ~ l e a v a g e . ~ % ~ ~ On Au(ll0) in vacuum, methanol does not dissociate prior to molecular desorption by 200 K.25 In the presence of preadsorbed oxygen, however, methanol reacts to form water, methyl formate, hydrogen, and carbon dioxide, although adsorbed carbon monoxide was not detected.25 It is conceivable, therefore, that similar Bronsted acidbase chemistry involving adsorbed oxygen species is at play in the present electrochemical system, although I ,
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Figure 2. As in Figure 1, but for 5 mM methanol.
rate is 10 mV s-l, each spectrum was therefore recorded within a 0.08-V potential span starting with the value indicated. The voltammetric potential excursion was from -0.3 to 1.5 V and return; the spectra are stacked in Figures 1 and 2 in a upward temporal Sequence (i.e. read from bottom to top). No significantvoltammetric oxidation of methanol could be discerned under these conditions, even for concentrations as high as 1M. Despite this lack of electrocatalytic activity, the spectra display several features, not present prior to the addition of methanol,that indicate a surprising ability of this speciesto undergo dissociative chemisorption on gold. Most prominently, a band at 2115-2145 cm-l is seen in both Figures 1and 2, which becomes more intense
(19) Tadayyoni, M. A.; Weaver, M. J. Langmuir 1986,2, 179. (20) (a) Juanto, S.; Beden, B.; Hahn, F.; Leger, J. M.; Lamy, C. J. Electroanol. Chem. 1987,237,117. (b)Iwasita, T.; Vielstich, W.; Santos, E. J. Electroanal. Chem. 1987,229,367. (c) Beden, B.; Hahn,F.; Leger, J. M.; Lamy, C.; Santos Lopes, M. I. D. J.Electroanol. Chem. 1989,258, 463. (d) Beden, B.; Hahn, F.; Lamy, C.; Leger, J. M.; de Tacconi, N. R.; Lenza, R. O.;Arvia, A. J. J.Electroanol. Chem. 1989,261,401. (e) Pham, M. C.; Moslih, J.; Siman, M.; Lacaze, P. C. J. Electroanal. Chem. 1990,
282,287. (0Beden, B.; Hahn, F.; Leger, J. M.; Lamy, C.; Perdriel, C. L.; de Tacconi, N.R.; Lema, P. 0.;Arvia, A. J. J. Electroanal. Chem. 1991,
301, 129. (21) (a) Willsau, J.; Wolter, 0.; Heitbaum, J. J. Electroanal. Chem. 1985,185,163. (b) Wilhelm, S.; Iwasita, T.; Vielstich, W. J.Electroanol. Chem. 1987,238,383. (c) Christensen, P. A.; Hamnett, A.; Weeks, S. A. J. Electroanal Chem. 1988,260, 127. (d) Cattaneo, B. B.; S a n h , E.; Vielstich, W.; Linke, U. Electrochim. Acta 1988, 33, 1499. (e) Iwasita, T.; Nart, F. C. J. ElectroanoZ. Chem. 1991,317, 291. (22) Chang, S.-C.; Hamelin, A.; Weaver, M. J. J. Chim.Phys. 1991,88, 1615. (23) Franaszczak, K.; Herrera, E.; Zelenay, P.; Wieckowski, A.; Wang, J.; Masel, R. I. J. Phys. Chem. 1992,96,8509. (24) (a) Gates, J. A.; Kesmodel, L. L. J.Catal. 1983,83,437. (b)Davis, J. L.; Barteau, M. A. Surf.Sci. 1990,235, 235. (25) Outka, D. A.; Madix, R. J. J. Am. Chem. SOC. 1987, 109, 1708.
1400 Langmuir, Vol. 9, No. 5, 1993 adsorbed 0 or OH is extremely unlikely to be present at the low potentials (0 V vs SCE) where adsorbed CO formation from methanol is observed (Figures 1and 2). Either the -CHO or -C-OH species may act as surface intermediates en route to -CO production (eq 1); both of these adsorbates are apparently present on the basis of the SER spectra in Figure 1. Significantly different spectral behavior was observed for methanol concentrations above ca. 1mM. As exemplified in Figure 2, besides the appearanceof vco and VM-co features as before, clear-cut bands are observed at 13781390,785,and 250-280 cm-l, while the -CHO and -C-OH features are now absent. These three new bands can each be attributed to adsorbed formate (HCOO)bound via both oxygens to the gold surface, specifically to symmetric 0-C-0 stretching (VOCO),04-0 bending (boco), and metal-formate stretching (VM-HCOO), respectively. These assignments are supported by vibrational (electronenergy loss spectroscopy) studies of methanol decomposition on Rh(111)Z" and formic acid adsorption on Pt(111),26b together with a SERS study of COZadsorption on copper and silver films.sb Attempts were made to corroborate the spectral assignments by means of 13C isotopic substitution but were thwarted by complications arising from impurities in the isotopically labeled methanol. As might be expected, the amounts of adsorbed CO formed increase toward higher methanol concentrations, as gleaned from the vco band intensities. More surprising is the apparently preferential production of adsorbed formate rather than -CHO or -COH under these conditions. Unlike the species shown in eq 1,which are formed via unimolecular surface decomposition, the production of formate involves reaction of one (or more) of these adsorbates with an oxygen donor. Indeed, the formate spectral features appear to high potentials, 20.8 V, close to where gold oxidation commences, as confirmed by the appearance of the characteristic broad band centered at ca. 560-570 cm-l (Figures1and 2),27suggestingthat surface hydroxyl acts as the oxygen donor. Nevertheless, the formation of -CHO, -COH, and CO is also seen to be triggered in a similar potential-dependent fashion (Figures 1and 2). The latter finding suggeststhat surfacehydroxyl may also assist the nominally unimolecular decomposition sequence in eq 1, possibly by providing sites where hydrogen transfer can take place, thereby aiding C-H bond cleavage. This phenomenon may well have a close relationship to the activation of methanol decomposition by preadsorbed oxygen observed on gold in vacuum.25 Regardless of such details, it is evident that gold provides an effective catalyst for methanol surface decomposition, and yet the adsorbed species once formed are unable to be effectively converted to COz or other electrooxidation products. These observations become more surprising (and interesting) when placed alongside the facile electrooxidation of carbon monoxide that is observed to occur on gold from CO solutions in acidic perchlorate medialnI@ Figure 3A shows SER spectral sequences in the vco (20002150 cm-l) as well as low-frequency (200-700 cm-l) regions obtained in the same fashion as Figures 1 and 2, during a cyclic voltammetric excursion from -0.3 to 1.5 V and return, for a saturated (ca. 1mM) solution of CO in 0.1 M HClOd. Severalfeatures of Figure 3A are of mechanistic interest in comparison with the corresponding behavior (26) (a) Houtman, C.; Barteau, M. A. Langmuir 1990, 6, 1558. (b) Columbia, M.R.; Crabtree, A. M.; Thiel, P. A. J . Am. Chem. SOC.1992, 114, 1231. (27) Desilvestro, J.; Weaver, M. J. J . Electroanol. Chem. 1986, 209, 377. (28) Farrugia, T. R.;Fredlein, R. A. Aust. J. Chem. 1984, 37, 2415.
Zhang and Weaver
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Figure 3. Selected members of potential-dependent SER spectral sequences obtained on gold-during vo1ta"ograms at 10 mV a-l for (A) CO-saturatedsolution in 0.1 M HC104and (B) 0.1 M HC104 only. Other details as in Figure 1. for methanol. Most noticeably, the relatively intense vco band seen at 2110-2120 cm-' during the early portion of the voltammetric excusion in Figure 3A is attenuated severely at potentials above 0.1 V and lost above 0.6 V, returning only at low potentials during the subsequent negative-goingsweep. This spectral behavior is consistent with the corresponding voltammograms for CO electrooxidation, which display an onset of anodic current at ca. 0.1 V leading to a diffusion-controlled peak located close to 0.5 V (cf. refs 17b and 19). Indeed, similar potentialdependent vco behavior has been obtained for CO voltammetric oxidation on some monocrystalline gold electrodes by means of real-time infrared spectroscopy.17b~c Clearly, then, the terminal adsorbed CO formed on gold from solution CO undergoes efficient electrooxidative removal even at low positive potentials. This behavior is in marked contrast to that of adsorbed CO produced by methanol decomposition on gold, which is formed preferentially only at high positive potentials and is apparently not readily electrooxidized. Possible clues to the reasons for this marked behavioral difference can also be gleaned from Figure 3. Prior to the complete electrooxidative removal of adsorbed CO in Figure 3A, the vco peak is seen to shift to significantly higher frequencies. This behavior was also observed in an earlier potential-dependent SERS study of the same system over longer time scales (10-30 min, as obliged by the use of a scanning Raman spectrometer) and ascribed to the formation of CO bound to partially oxidized gold sites.lS This interpretation is prompted by the higher vco frequencies, signaling the presence of increased m e t a l 4 0 u bonding as would be anticipated at electron-deficient sites brought about by juxtaposition of hydroxyl or other electronegative adsorbates. Such surface chemistry can readily account for the observed remarkably facile electrooxidation of CO to COz on gold, since adjacent coadsorption of oxygen-containing species is a likely prerequisite for reaction. Nonetheless, the Surprising feature is the occurrence of such electrooxidation, along with apparent oxygen-donor coadsorption, at potentials far below those required for voltammetric
Decomposition of Methanol on Gold
surface oxidation in 0.1 M HC104, ca. 0.9 V. Further inspection of the low-frequency region of Figure 3A, however, provides some evidencethat CO adsorption itself may induce the O-donor coadsorption conducive to CO electrooxidation. Thus the band at 465 cm-l, assigned to a metal-carbon stretch, is accompanied by a satellite peak at 420 cm-l, suggestive of a surface-oxygen vibration (6. ref 27). Neither of these features is observed on gold in 0.1M HC104 alone, as shown by the corresponding ‘blank” spectral sequence in Figure 3B. Furthermore, an intense band centered at 525 cm-l is present even following electroreductive removal of the gold oxide during the subsequent negative-going potential sweep, below 0.8 V, in CO-saturated HC104 (Figure 3A). This band is not observed in the absence of CO, only the broad feature at higher frequencies due to anodic oxide being obtained at more positive potentials (Figure 3B, cf. ref 27). While clear-cut interpretation of these additional spectral features is not feasible here, they nonetheless are definitely suggestive that additional adsorbed species are generated over a wide potential range by the presence of solution
Langmuir, Vol. 9, No.5, 1993 1401
%
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Significantly,these additional features are not evident in the potential-dependent spectra obtained in the presence of methanol (Figures 1 and 2). It is therefore reasonable to conclude that the inability of the adsorbed CO formed by methanol decomposition to undergo electrooxidation on gold arises from the presence of additional surface species that prevent adsorption of the required oxygen-donor species. A similarargumentcan also account for the observed impotency of the other chemisorbed species formed from methanol on gold to act as adsorbed intermediates for methanol electrooxidation in this environment. Unlike acidic media, methanol is observed to undergo significant voltammetric electrooxidation in alkaline solutions. Few clear-cut SER spectral features, however, could be discerned under the latter conditions. Nevertheless, adsorbed CO is seen to be formed even at the initial potential, -0.5 V vs SCE. Sweeping the potential in the positive direction yields a marked diminution in the vco signal intensity (at 2150-2110 cm-l) close to the onset of methanol electrooxidation. Similar to the behavior in acidic media, however, considerably more adsorbed CO is seen to be formed during the subsequent negative-going potential sweep, following the initial reductive removal of the oxide film.Typical data illustrating this point are displayed in Figure 4A in the form of a plot of the vco band intensity (circles) versus the electrode potential, for a potential excursion from -0.8 to 0.6 V and return at 10mV s-1 (the open and filled circles refer to the forward and reverse sweeps, respectively). The corresponding voltammogram, obtained simultaneously,is shown underneath (B)as the solid trace; the dashed trace was obtained in the absence of methanol. Formic Acid on Gold. In contrast to methanol, formic acid is observed to undergo relatively facile electrooxidation in perchloric acid media, yielding significant anodic current above about 0.3 V in 0.1 M HC104.17a Figure 5 shows members of a typical SER spectral sequence obtained during a potential excursion from -0.3 to 1.5 V and return at 10 mV s-1 on gold in 0.1M HC104 containing 5 mM formic acid. The two frequency regions shown, 2050-2100 and 200-800 cm-l, enable the potential-dependent occurrence of adsorbed CO and formate to be followed. Similarly to the methanol case, adsorbed CO is formed chiefly only at high potential during the reverse potential sweep. By contrast, bands at ca. 280-290 and
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E / V vs. SCE Figure 4 (A) Intensity of YCO band (arbitrary scale) versus electrodepotential extracted from SER spectra on gold in 0.1 M KOH with 0.1 M methanol, during 10 mV 8-1 potential sweep from -0.8 to 0.6 V and return. (The open and filled circles refer to the forward and reverse sweeps, respectively.) (B)Corresponding voltammograms obtained in presence (solidtrace) and absence (dashed trace) of 0.1 M methanol.
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Figure 5. Similarly to Figure 1, but for 0.1 M HClO, containing 5 mM formic acid.
765 cm-l, characteristicof adsorbed formate,appear during the initial positive-going sweepwithin the potential range, 0.5-0.9 V, where formic acid electrooxidation (to COz) is observed to occur (Figure 5). It is therefore reasonable to conclude, perhaps not surprisingly,that adsorbed formate is the likely adsorbed intermediate for this reaction. The relatively small amounts of adsorbedCO formed, together with ita absence during the initial poaitive-going sweep where the electrooxidation reaction occurs, suggest that this adsorbate plays little or no role in the electrocatalyais.
Zhang and Weaver
1402 Langmuir, Vol. 9, No. 5, 1993
v:
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E
Figure 6. Similar to Figure 1, but for platinum-coated gold electrode in 0.1 M HC104containing 0.5 M methanol, for 10 mV 8-1 potential sweep from -0.3 to 1.3 V and return.
In 0.1 M KOH, formic acid (now present, of course, as formate) does not appear to undergo detectable electrooxidation on gold. Nevertheless, adsorbed CO is seen from SERS to form readily even at initial potentials as negative as 4 . 8 V. The adsorbed CO appears to be nonreactive, in that the vco SERS signals are largely potential independent. In addition to methanol and formic acid, a more cursory examination was made of the C2 reactants ethanol and acetaldehyde on gold in 0.1 M HClOd. The latter reactant exhibited anodic current starting at 0.6 V, while the former species yielded no detectable electrooxidation under these conditions. In neither case, however,were clear-cut SERS bands observed throughout the voltammetric potential excursion other than those due to gold surface oxidation. Methanol and Formic Acid on Platinum-Coated Gold. While the adsorptive and electrocatalyticproperties of gold are certainly of interest and significance, of more widespread concern is the elucidationof surface speciation for organic electrooxidations on platinum-group metals. As already mentioned, in suitable cases SERS can be obtained on transition metals such as platinum by depositing them as ultrathin films on gold. Our initial application, as well as demonstration, of this tactic was to carbon monoxide adsorption (from solution CO) on platinum, palladium, rhodium, and ruthenium film electrodes.12 Carbon monoxide provides a especially suitable test adsorbate since the vco frequenciesare well-known to be sensitive to the nature of the metal as well as the binding geometries, enabling a ready spectral distinction between CO bound to the overlayer as compared with residual gold sites.laa Our subsequent published applications of the approach have emphasized chiefly adsorption from the gas phase at ambient and elevated temperatures, where spectral interferences from adsorbates present at residual gold sites can commonly be neg1e~ted.l~ Nevertheless, the possibility of utilizing suchtransitionmetal film surfaces to explore the nature of adsorbates present during electrocatalyticprocesses by means of realtime SERSremains an attractive prospect in suitable cases. As an illustration, Figure 6 shows selected members of a
typical sequence of SER spectra obtained on platinumcoated gold in 0.1 M HC104 containing 0.5 M methanol. The spectra were obtained similarly to those described above, during a voltammetricexcursion at 10mV s-1from 4 . 3 to 1.3V and return. The voltammograms indicated the occurrence of facile electrooxidation of methanol in a similar fashionto bulk polycrystallineplatinum, displaying anonset of anodic current at ca. 0.35 V. Essentially similar potential-dependent spectral features were observed over the methanol concentration range 5 mM to 1 M. A surprising aspect of the potential-dependent SER spectra is the presence of only a very weak uco band (not shown in Figure 6) at frequencies, 2030-2050 cm-l, expected for terminally bound CO on platinum electrodes. The weak 495-cm-1 band seen in Figure 6 is attributed to the metal-carbon (upgo) stretch for adsorbed CO. (The broader feature centered at 580 cm-l, appearing at high positive potentials, is due to platinum oxide formation.? In contrast, strong uco bands are commonly observed by means of infrared spectroscopy during methanol electrooxidation under similar conditions on bulk polycrystalline surfaces (see for example, refs 4, 8, and 29). However, real-time infrared spectra obtained in our laboratory during methanolelectrooxidation on platinumcoated gold electrodes under near-identical conditions to those in Figure 6 also display extremely weak uco bands. (The infrared experiment was performed essentially as described in refs 4 and 8.) Following the procedure outlined in ref 4, the fractional CO coverage, Bco, was ascertained from the infrared vco intensities to be very low, S0.05, at potentials in the vicinity of where methanol electrooxidationproceeds. Apparently, then, the present Pt-film surface does not efficiently decompose methanol to yield CO, at least as a final product. Nevertheless, the Pt-coated surfaces are readily able to bind CO strongly, yielding saturation coverages from CO-containing solutions, as indicated from the intense SERS as well as infrared uco bands obtained under these ~ o n d i t i o n s . ' ~ ~ * ~ ~ Interestingly,several in situ infrared studies of methanol on various dispersed or roughened platinum electrodes also show a paucity of adsorbed CO formation in comparison with conventionally polished platinum surfaces.20dpf*21c The present SERS data nonetheless suggest the presence of at least one other adsorbed poison present on the Pt film surface under methanolreaction conditions. Inspection of Figure 6 reveals the presence of a strong band at 2280-2290 cm-l that is removed at potentials, ca. 0.6-0.7 V, close to the onset of Pt surface oxide formation on the initial positive-goingsweep, reappearing following oxide reduction on the reverse sweep. The high frequency of this band is suggestive of an alkyne-like species. Although a bona fide assignment cannot be made on the basis of Figure 6alone, one distinct possibility is acetylenic species formed by processes such as31
-
CH30H + COad CH3COOH
-
C H e H + H20, (2)
Support for this contention is provided by the observation of a comparable band at 2285 cm-' on platinum-coated gold in 0.1 M HClOr containing acetic acid, although comparison of Figure 6 with SER spectra obtained for alkynes32 yields no clear-cut conclusions. (29) h t h , J. D.; Weaver, M. J. J.Electroanul. Chem. 1991,307,119. (30) Zhang, Y.; Weaver, M. J. J. Electroanul. Chem., in press. (31) Roth, J. F.; Craddock, J. H.; Herschman, A.; Paulik, F. E. Chem. Tech. 1971, I, 600. (32) (a) Patterson, M. L.; Weaver, M. J. J. Phys. Chem. 1985,89,5046. (b) Feilchenfeld, H.; Weaver, M. J. J. Phys. Chem. 1989, 93, 4278. (c) Feilchenfeld, H.; Weaver, M. J. J. Phys. Chem. 1991, 95,7771. (33) Koel, B. E.; Sellidj, A.; Paffett, M. T. Phys. Rev. B 1992,46,7846.
Decomposition of Methanol on Gold
With regard to the microscopic state of the platinum f i i s on gold, it is worth noting that strong evidence has recently been presented for surface alloy formation in ultrathin films of palladium deposited on Au(ll1) at 300 K and higher temperatures in vacuum.33 We certainly cannot eliminate the possible occurrence of surface alloying in the present electrodepositedplatinum films, which may indeed play a role in the methanol dissociation chemistry. However, a number of electrochemical properties of the platinum films, including voltammetric oxide formation and removal, and CO electrooxidation,match closely with the observed behavior on conventional polycrystalline platinum electrode^.^^^^^ Similarly to unmodified gold in methanol-containing 0.1 M HC104, platinum-coated gold displayed a band at 1365 cm-l ascribed to voco for adsorbed formate (Figure 6). A high-frequency partner was also observed at 2930 cm-l (Figure6),the frequency being consistent with a C-H vibration (VC-H) associated with an sp3carbon (cf. ref 34). While it is tempting to assign this feature to VC-H for adsorbed formate, the band was not observed for methanol solutions on unmodified gold. It is appropriate to mention here an inherent difficulty with such SERS measurements, especiallyon the transition-metal-modifiesurfaca: d trace solution impurities can readily yield vibrational bands, especially in the ca. 1000-1700 cm-l range. Even in the receipt of "clean" blank spectra, then, it is always possible that additional weak SER features can arise from impurities added along with the solute species of interest. Corresponding potential-dependent SER spectra were also obtained on platinum-coated gold for solutions of formic acid in 0.1 M HClOd. In contrast to methanol, formic acid decomposes readily to form CO, as seen from a clear-cut vco band at 200&2030 cm-l along with a v p ~ o feature at 480-500 cm-l. These bands are attenuated at potentials above 0.8 V where Pt surface oxidation occurs, the latter being evidenced by a broad feature at 530-570 cm-l. This behavior is roughly consistent with potentialdependent infrared spectra obtained for formic acid electrooxidation on bulk polycrystalline platinum.lT8 No additional clear-cut bands were obtained in the SER spectra, however, which would assist the identification of the reaction intermediate(s). Concluding Remarks: Utility and Limitations of SERS in Electrocatalytic Studies. As might be surmised from the foregoing description, the present study brings to the fore some limitations as well as advantages of SERS as an in situ probe of surfacespeciation in organic electrocatalyticprocesses. In some respects, the relatively high sensitivity, freedom from bulk-solution spectral interferences, and the detection of vibrational bands over wide frequency ranges that are commonlytouted as virtues of SERSalso prove to be of substantial merit for thepresent type of system. The ability of acquire real-time spectral sequences under conventionalvoltammetricconditions of relevance to electrocatalyticsystem is also of major value. The present finding that methanol can undergo decomposition on gold in acidic media to yield chemisorbed (34) Ibach, H.; Hopster, H.; Sexton, B. Appl. Surf. Sci. 1977, 1, 1.
Langmuir, Vol. 9, No. 5, 1993 1403
fragments that are commonly claimed to be reactive intermediates in transition-metal electrocatalyticsystems is as unexpected as it is interesting. In lieu of such information, the observation that methanol does not undergo electrooxidation on gold in acidic media would be reasonably ascribed to the anticipated inability of this interface to induce the C-H (and O-H) band cleavage necessary for electrocatalysisto proceed. By contrast, the SERS data oblige us to reach a completely different conclusion, that such molecular fragmentation is a necessary but not sufficientcondition for electrocatalysis.The key additional requirement, which is fulfilledhandsomely for CO electrooxidation on gold, but which is apparently lacking for methanol, is the ready availabilityof the oxygendonor coadsorbates with which the chemisorbed intermediates can be converted to COP and other products. The SERS data for CO electrooxidation are also seen to corroborate directly this latter assertion. Indeed, the ability of gold to electrooxidize formic acid but not methanol to COZin perchloric acid media may well reflect the lack of a required oxygen-atom-transfer step in the former case. Nevertheless, several interpretative difficulties are evident upon reflection. More so than with surface infrared spectroscopy, the extraction even of approximate surface concentrations from SERS intensities is notoriously difficult (and ambiguous), so that the detection of surface decomposition processes by these means alone seldom provides assurance that such chemistry is predominant. A related limitation is that the Raman scattering cross sections (aswell asSERS enhancementfactors) are often expected to be low for aliphatic organic molecules, as we are concerned with here, so that substantial adsorptivechemistry may go unnoticed in the SER spectra. Admittedly, this latter difficulty is also often faced in applications of infrared spectroscopy but can be problematical for somewhat different reasons with SERS. The examination of adsorptive chemistry on transition-metal film electrodesfaces an additional complication along these lines in that adsorbed species present on transition-metal overlayer sites probably display smaller SER enhancement factors than on unmodified gold, thereby exacerbating their detection and distinction from species bound to residual gold sites. These and related difficultiesnotwithstanding,however, the present findings enable one to remain cautiously optimistic concerning the potential value of SERS for eluciding electrocatalytic phenomena. When applied to suitable systems, and especially when employed along with other spectroscopic methods and in concert with conventional electrochemical information, the SERS technique can provide the additional insight into surface speciation that is still sorely needed for a fundamental appreciation of electrocatalysis. Acknowledgment. This work is supported by the National Science Foundation.
